Chapter 2 Introduction of MB-OFDM proposal
2.1 Introduction to MB-OFDM architecture
In this section a brief introduction of an UWB system intended for wireless personal area network (WPAN) is presented. It uses the modulation based on MB-OFDM for conveying information over relatively short distances among few participants. Every device of the communication system interacts using the concept of piconets. A piconet is a wireless ad hoc data communications system which allows a number of independent data devices (DEVs) to communicate with each other. An 802.15.3a piconet consists of several components, as shown in figure 2.1.1. The basic component is the DEV. One DEV is required to assume the role of the piconet coordinator (PNC) of the piconet. The PNC provides the basic timing for the piconet with the beacon. Additionally, the PNC manages the quality of service (QoS) requirements, power save modes and access control to the piconet.
figure 2.1.1 piconet elements (source : 802.15.3 standard) .
WPAN is a packet base transmitting communication system. A packet is composed with couples of OFDM symbol. It uniformly cuts 528 MHz to 128 subcarriers, 4.125 MHz for each subcarrier, in which 100 subcarriers are for information tones, 12 subcarriers are for pilot tones, 10 subcarriers are for guard tones, and 6 subcarriers are for null tones. The relative location in frequency domain is shown in figure 2.1.3. The time and frequency domain transform is implemented using Fast Fourier Transform/ Inverse Fourier Transform (FFT/IFFT) (figure 2.1.2), and the FFT/IFFT period is 242.42 ns. In each OFDM symbol, there are 32 sampling intervals for cyclic prefix extension and 5 sampling intervals for guard intervals which are used for frequency hopping. So it takes 312.5 ns to transmit an OFDM symbol. The relative parameters are summarized in table 2.1.1.
figure 2.1.2 input and output of IFFT
Parameter Value NSD: Number of data subcarriers 100
NSDP: Number of defined pilot carriers 12 NSG: Number of guard carriers 10
NST: Number of total subcarriers used 122 (= NSD + NSDP + NSG)
∆F: Subcarrier frequency spacing 4.125 MHz (= 528 MHz/128)
TFFT: IFFT/FFT period 242.42 ns (1/∆F) TCP: Cyclic prefix duration 60.61 ns (= 32/528 MHz) TGI: Guard interval duration 9.47 ns (= 5/528 MHz)
TSYM: Symbol interval 312.5 ns (TCP + TFFT + TGI)
Table 2.1.1 Time relative parameters of MB-OFDM
figure 2.1.3 subcarrier frequency allocation (source : 802.15.3a proposal )
In each OFDM symbol, twelve pilot signals are for the purpose of making coherent detection robust against frequency offsets and phase noise. These pilot signals shall be put in subcarriers numbered –55, –45, –35, –25, –15 –5, 5, 15, 25, 35, 45, and 55. The contribution due to the pilot subcarriers for the kth OFDM symbol is given by the inverse Fourier Transform of the sequence Pn
below, which is further BPSK modulated by a pseudo-random binary sequence,
pl (defined further below), to prevent the generation of spectral lines.
For modes with data rates less than 106.67 Mbps:
55
For 106.67 Mbps and all higher rate modes:
55
The polarity of the pilot subcarriers is controlled by the following pseudo-random LFSR sequence, pl:
p0…126 = {1, 1, 1, 1, -1, -1, -1, 1, -1, -1, -1, -1, 1, 1, -1, 1, -1, -1, 1, 1, -1, 1, 1, -1, 1, 1, 1, 1, 1, 1, -1, 1, 1, 1, -1, 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, 1, -1, 1, -1, -1, 1, -1, -1, 1, 1, 1, 1, 1, -1, -1, 1, 1, -1, -1, 1, -1, 1, -1, 1, 1, -1, -1, -1, 1, 1, -1, -1, -1, -1, 1, -1, -1, 1, -1, 1, 1, 1, 1, -1, 1, -1, 1, -1, 1, -1, -1, -1, -1, -1, 1, -1, 1, 1, -1, 1, -1, 1, 1, 1, -1, -1, 1, -1, -1, -1, 1, 1, 1, -1, -1, -1, -1, -1, -1, -1}
MB-OFDM WPAN provides 53.3, 80, 110, 160, 200, 320, 400, and 480 Mb/s data rates. Unlike 802.11a, those rates are archived by changing the coding rate of convolutional code and the spreading factors instead of by modulation. The modulation used in this proposal is only QPSK. The relative between the data rate and the parameters is as shown in table 2.1.2. Where the conjugate symmetric input to IFFT means that data is just carried by the tone with positive index, and the tones with negative index take the values conjugate to that of
positive indexes symmetrically. And the time spreading is implemented by transmitting the same OFDM twice.
Data Rate (Mb/s)
Modulation Coding rate (R)
Conjugate Symmetric Input to
IFFT
Time Spreading
Factor
Overall Spreading
Gain
Coded bits per OFDM symbol (NCBPS)
53.3 QPSK 1/3 Yes 2 4 100
80 QPSK ½ Yes 2 4 100
110 QPSK 11/32 No 2 2 200
160 QPSK ½ No 2 2 200
200 QPSK 5/8 No 2 2 200
320 QPSK ½ No 1 (No
spreading)
1 200
400 QPSK 5/8 No 1 (No
spreading)
1 200
480 QPSK ¾ No 1 (No
spreading)
1 200
Table 2.1.2 Rate-dependent parameters
The WPAN PHY operates in 3.1 – 10.6 GHz frequency as regulated by FCC.
The relation between center frequency and band number is given by the equation:
Band center frequency = 2904+528×nb,nb =1...14(MHz).
Based on this definition, five band groups are defined, consisting four groups of tree bands each and one group of two bands. Band group 1 is used for Mode 1 devices (mandatory mode). The remaining band groups are reserved for future use. The band allocation is summarized in table 2.1.3. The transmitted signal power in each band is limited by the power spectrum density (PSD) mask (figure 2.1.4) regulated by FCC.
Band Group BAND_ID Lower frequency Center frequency Upper frequency 1 3168 MHz 3432 MHz 3696 MHz 2 3696 MHz 3960 MHz 4224 MHz 1
3 4224 MHz 4488 MHz 4752 MHz 4 4752 MHz 5016 MHz 5280 MHz 5 5280 MHz 5544 MHz 5808 MHz 2
6 5808 MHz 6072 MHz 6336 MHz 7 6336 MHz 6600 MHz 6864 MHz 8 6864 MHz 7128 MHz 7392 MHz 3
9 7392 MHz 7656 MHz 7920 MHz 10 7920 MHz 8184 MHz 8448 MHz 11 8448 MHz 8712 MHz 8976 MHz 4
12 8976 MHz 9240 MHz 9504 MHz 13 9504 MHz 9768 MHz 10032 MHz 5
14 10032 MHz 10296 MHz 10560 MHz
Table 2.1.3 OFDM PHY band allocation
figure 2.1.4 transmit power spectrum density mask
In 802.15.3a standard, the maximum center frequency offset (CFO) of transmitter and receiver is ±20ppm. In operation mode 1, operated with center frequency 4488 MHz and relative oscillator offset is ±40ppm, the maximum CFO sensed by receiver is 0.17952 MHz ( about 0.04 of the distance between subcarriers).
MB-OFDM preamble composed of tree kinds of training sequence: (1) packet synchronization sequence (defined in time domain), (2) frame synchronization sequence (defined in time domain), and (3) channel estimation sequence (defined in frequency domain).
Unique logical channels corresponding to different piconets are defined by using up to four different time-frequency codes for each band group. Different preamble patterns are used in conjunction with the different time-frequency
codes. The time-frequency codes and associated preamble patterns for Mode 1 devices (which operate in band group 1) are defined in table 2.1.4.
Channel Number
Preamble Pattern
Mode 1: Length 6 Time Frequency Code
1 1 1 2 3 1 2 3
2 2 1 3 2 1 3 2
3 3 1 1 2 2 3 3
4 4 1 1 3 3 2 2
5 1 1 2 1 2 1 2
6 2 1 1 1 2 2 2
Table 2.1.4 time frequency code
MB-OFDM use zero padding instead of cyclic extension. The reason for using zero padding is that adding cyclic extension in an OFDM symbol makes the autocorrection of the sequence raise high at specific moment. PSD is the fourier transform of autocorrection function, and the raising of autocorrection function makes the PSD of the transmitted signal ripple. Zero padding has no such problem, and it also provides the same robustness to multipath fading as cyclic extension. But a little modification should be made in order to eliminate the inter-carrier interference due to the inaccuracy of symbol timing. The modification name “overlap adding” is adding the signal spreading from the FFT input to the front part of the signal (figure 2.1.5). This modification makes the zero-padding signal looks like the cyclic prefix one.
figure 2.1.5 The overlap adding of received signal